The present invention generally relates to laboratory work enclosures having an isolated work space and more particularly to biological safety cabinets constructed to prevent airborne contaminants within the work space from escaping from the cabinet into the ambient environment.
In the past safety cabinets have been developed for protecting a technician working with various toxic and hazardous materials such as biological matter and radiological materials from exposure to airborne contaminants generated during the handling of these materials. Past work stations have been developed with the object of completely sealing off the work area within the safety cabinet from passage of contaminated air both into the work area and out of the work area into the ambient environment.
The containment or isolation of hazardous and toxic laboratory substances has generally been accomplished by providing a work area which is enclosed or covered with a hood structure having one or more access openings to the work area. The access openings allow a technician, for example, to reach into the work area to handle the material contained in the hood structure. Since these access openings provide another avenue for transfer of hazardous and toxic airborne contaminants between the inside of the hood structure and the outside or ambient environment, it has been a well known past practice to provide a means for causing a continuous, positive air flow into the hood structure through the access opening or openings. This continuous flow of air from the ambient environment through the access opening or openings prevents the escape of any airborne contaminants from the work area.
Many prior safety cabinet structures include high efficiency particulate air (HEPA) filters for filtering air being directed into the work area and air being exhausted from the cabinet into the ambient environment. The air being directed into the work area is drawn both from air recirculated from the work area and air taken in from the ambient environment through the access opening or openings. Thus, the HEPA filter which filters air entering the work area (the "supply filter") ensures that any contaminants picked up from the work area are not recirculated back into the work area and further that contaminants from the ambient air are not circulated through the work area. The HEPA filter which filters air exhausted from the cabinet (the "exhaust filter") ensures that hazardous and toxic airborne contaminants generated by the material contained in the cabinet are not exhausted into the ambient environment. Many other systems requiring very clean air utilize only one HEPA filter which may, for example, simply filter recirculated air within the system.
Whether one or more HEPA filters are used in the system, it has also been generally known to employ one or more static pressure gauges to measure the air pressure across the filters. Where one static pressure gauge is used in a system utilizing two HEPA filters such as the supply and exhaust filters as described above, the pressure differential across the two HEPA filters is measured to obtain information as to the loading of the filters. As the filters become loaded, the resistance to air pressure increases and the reading on the static pressure gauge correspondingly increases. When the reading on the static pressure gauge increases by a given amount, e.g., 50% higher than an original measurement taken with clean filters, the filters usually need replacement due to loading inefficiencies. A similar process for determining the loading of the supply and exhaust filters may be used with two pressure gauges by utilizing each pressure gauge to measure the pressure across a different filter.
Two illustrative examples of safety cabinet structures of the above-mentioned type are found in U.S. Pat. No. 3,895,570 issued to Eagleson, Jr. on Jul. 22, 1975 and U.S. Pat. No. 4,637,301 issued to Shields on Jan. 20, 1987. Each of these patents disclose work stations or safety cabinets of the general type described above having one or more HEPA filters for cleaning air which is circulated through the cabinet structure. The patents to Eagleson and Shields further disclose the use of conventional pressure gauges for measuring the pressure across the HEPA filters.
While past safety cabinets have generally accomplished the goal of preventing airborne contaminants within the cabinet structure from reaching the ambient environment through either the access opening or other seams in the cabinet structure, it has been found through halogen leak testing that airborne contaminants can also leak through the conventional static pressure gauges used in these cabinets. These pressure gauges are exposed to the contaminated air within the cabinet and are not designed to seal airborne particles 0.3 microns or greater in diameter within the cabinet structure as would be required to meet recognized industry safety standards.